As known, a classical Doherty amplifier has two amplifying devices arranged in parallel and of the same power capability. The first one of the devices (main stage) operates in a class-AB amplifier mode and the second one (peak stage) operates in a class-C amplifier mode. These devices are separated at their inputs and at their outputs by 90° phase-shifting networks. The output phase-shifting network has a specific characteristic impedance Z0 which must be equal to the optimal load impedance RLm of the main stage. The input signal is split so as to drive the two amplifiers, and a summing network, known as an “impedance inverter” or a “Doherty combiner”, is operative to: a) combine the two output signals, b) to correct for phase differences between the two output signals, and c) to provide an inverted impedance at the output of the Doherty amplifier with respect to the impedance as seen from the output of the main stage. Due to the impedance inversion, the main stage operates at load when the input signal level is low, the load then being two times higher than the optimal load RLm=2 Z0 which allows higher power efficiency of the main stage, and also of the Doherty amplifier, while the peak stage remains inactive. The double load at the output of the main stage is possible due to proper arrangement of the output load RLD of the Doherty amplifier which, for the classical case, is RLD=½ Z0=½ RLm and which is transformed by the output phase-shifting network to RLm=4 RLD. When the input signal to the Doherty amplifier exceeds a certain level, the output level of the main stage reaches the maximum allowed amplitude and maximum power efficiency and the peak stage is activated and takes over the amplification. Above this level, the load impedance as seen by the main stage drops gradually with growing power level until it reaches its optimal value Z0, which occurs at the peak power level of the Doherty amplifier.
The classic Doherty amplifier is a so-called 2-way amplifier with a main stage and a peak stage. A multi-way (or N-way) Doherty amplifier has a main device and a plurality of peak devices operating in parallel. Advantages of a multi-way Doherty system are well known and are attributed to the increased range of load line control at the output of the main stage. In case of a 2-way symmetrical Doherty amplifier, the load seen by the main stage varies by a factor of two. In case of a classical 3-way Doherty amplifier, this variation is twice larger, and varies by a factor of four. This allows a similar improvement of efficiency, not at 6 dB back-off as for a 2-way Doherty amplifier, but at 12 dB of power back-off, which is currently demanded by new communication systems such as WiMAX (Worldwide Interoperability for Microwave Access). A classical 3-way Doherty requires quarter-wave-length lines between the outputs of the main stage and its first peak stage and also between the outputs of the first and additional peak stages. This makes the design of such a Doherty system very complicated. In addition, such design requires a large space in order to accommodate it, and mass-production samples can be predicted as showing very inconsistent behavior.
A practical implementation is a three-way Doherty amplifier. Examples of N-way Doherty amplifiers are described in, e.g., the following publications: N. Shrirattana et al, “Analysis and Design of High Efficiency Multistage Doherty Power Amplifier for Wireless Communications”, IEEE Transactions on Microwave Theory and Techniques, Vol. 53, No. 3, March 2005; I. Kim et al., “Highly Linear Three-Way Doherty Amplifier with Uneven Power Drive for Repeater System” IEEE Microwave and Wireless Components Letters, Vol. 16, No. 4, April 2006; I. Blednov and J. van der Zanden, “High-power LDMOS Integrated Doherty Amplifier for WCDMA”, IEEE Radio Frequency Integrated Circuits (RFIC) Symposium, June, 2006. Also see, e.g., U.S. Pat. No. 7,078,976 (Blednov); U.S. Pat. No. 6,853,245 and US published patent application US 20030141933.